Effect of the Fe3+/Cu2+ ratio on the removal of the recalcitrant oxalic and oxamic acids by electro-Fenton and solar photoelectro-Fenton

• Different photoactivity and oxidability of copper- and iron-carboxylate complexes.
• Synergistic effect between solar irradiation and electrochemical process.
• Faster removal of oxalic acid with 50/50 Fe3+/Cu2+ co-catalysts mixture by SPEF.
• Faster removal of oxamic acid with 75/25 Fe3+/Cu2+ co-catalysts mixture by SPEF.
• Larger generation of NO3− ion and volatile N-species at high current densities.

The degradation of 100 mL of 2.08 mM oxalic acid (OXL) and oxamic acid (OXM) solutions in 0.10 M Na2SO4 at pH 3.0 has been studied by solar photolysis (SP), electro-Fenton (EF) and solar photoelectro-Fenton (SPEF). EF and SPEF experiments were performed with a stirred electrochemical cell containing a 3 cm2 boron-doped diamond (BDD) anode and a 3 cm2 air-diffusion cathode that generates H2O2. Natural sunlight was directly exposed to the solution in SP and SPEF. Catalytic contents of 0.50 mM Fe3+, 0.50 mM Cu2+ or mixtures up to 0.50 mM of both metallic ions were added to the solution and a current density of 33.3 mA cm−2 was applied in EF and SPEF. OXM presented a remarkable slower decay than OXL by SP due to the lower photoactivity of metallic-oxamate complexes. OXL concentration decayed 90% in the presence of Fe3+ and Fe3+/Cu2+ mixtures, whereas the OXM drop decreased directly with increasing Cu2+ concentration. In EF, OXL was more slowly removed than OXM due to higher recalcitrant character of the Fe(III)-oxalate complexes which can only be mineralized by OH formed at the anode surface but not by those generated from Fenton’s reaction in the bulk. Upon Cu2+ addition, higher removal percentages were found because Cu(II)-carboxylate complexes are attacked by OH, thus accelerating the mineralization. In contrast, OXL destruction was largely enhanced in SPEF using the mixture of catalysts as a result of the photolysis of Fe(III)-oxalate complexes and the parallel mineralization of Cu(II)-carboxylate complexes by the high quantity of oxidant OH induced from photolysis of Fe(III)-aquo species. In all cases, the decay of OXL and OXM concentration obeyed a pseudo-first-order reaction, with an apparent rate constant dependent on the applied current in EF and SPEF.